The invention relates to a device for separating water and for filtering impurities out of fuel, to a fuel supply system including such a filtering device and to a Diesel engine with common rail fuel injection and a fuel supply system including the filtering device.
In internal combustion engines with fuel injection, water intermixed to a greater or lesser extent with the fuel causes damage to structural parts which may result in the failure of fuel injection components.
The free water contained in the fuel results from                water which is dissolved in the fuel and is separated as a function of temperature;        condensation water which is released from the air during the cooling phase after the engine has been stopped, the air originally having flowed into the fuel tank via the tank venting valve for the compensation of fuel consumption;        refueling, during which water is in most instances inadvertently introduced.        
Manufacturers of “common-rail injection systems” are meanwhile required to have a separation rate of >95% for the free water contained in the fuel. This places stringent requirements on the efficiency of such water separation system.
DE 31 45 964 A1discloses a device for separating water and for filtering impurities out of liquid hydrocarbons. A filter/water separator include filter and coalescer elements which are arranged horizontally in a housing and through which the flow passes from the inside outwardly, and separation elements, through which the flow passes from the outside inwardly. The separation elements have arranged inside them coaxially in each case a hollow cylinder which extends with its one end into the outlet and, at its other end, has an inflow port. The hollow cylinder is provided with lateral inlet ports which are distributed over its length and the overall cross section of which increases toward the outlet. The flow impinges on the separation elements obliquely; a flow component directed toward an end-face calming zone transports separated water drops toward the calming zone where the water drops fall into a water collection sump.
In versions of this kind, water separation normally takes place on the suction side of the fuel feed pump. The water separator is therefore arranged between the tank and the fuel feed pump. The water admixed with the fuel is present in relatively large drops, this constituting a precondition for this type of water separation. Water separation takes place, in the first step, during passage through a horizontally arranged coalescer element. In order to achieve guaranteed water separation, a likewise horizontally arranged cylindrical separation element is provided, which, in addition to the functionally necessary mesh width, has an impregnated hydrophobic outer surface which has an inhibiting effect on the passage of water. During operation, the small water drops are combined in the coalescer element into larger drops. When the water drops flow further on, they sink downward, offset at 90° to the fuel flow, due to their greater weight. Small and therefore light water drops are entrained and are caught on the surface of the separation element, that is to say they are held in contact with the separation element by the fuel flowing through the separation element and, under certain circumstances, are pressed through the separation element. This effect is to be reduced by means of a modified hollow cylinder which is provided in the separation element and is in the form of bores which are increased in cross section toward the outlet side.
A similar device is normally employed nowadays in vehicles with internal combustion engines. The separation element used here is modified in that the outer surface must have hydrophobic properties. Due to the higher fraction of impurities in the fuel, however, the hydrophobic properties are greatly reduced because of the increasing dirt load. This leads to an impairment in the separation behavior of water. Furthermore, basically, the through-flow resistance rises over the period of use on account of the increasing dirt load. This has a direct lessening effect on the delivery capacity of the low-pressure fuel feed pump and therefore on the fuel supply for the injection system of the engine. A version of this type basically has higher system sensitivity. Sometimes, this can be compensated for by a correspondingly larger design of the fuel feed pump, but this entails extra cost. Furthermore, above all, the effectiveness of water separation decreases rapidly with the increasing dirt load. This results in a markedly-reduced useful life, that is to say, in shorter service intervals, and therefore in an increase in operating costs. Enlarging the filter surface in order to compensate for this disadvantage requires a larger construction space, thus leading, in turn, to a rise in cost of the system.
It is also a disadvantage that the separated water which has dropped into the water collection space arranged underneath the water separator can be discharged manually only when the engine is at a standstill. For this purpose, discharge requires an airflow (ventilation) into the water separator. This then has to be de-aerated again by means of a hand-operated feed pump or the like and a manually actuated vent screw, in order to restore the satisfactory functioning of the injection system. Here, automation would require, as an additional disadvantage, the installation of a further pump.
GB 903 505 A, U.S. Pat. No. 4,579,652 and EP-A-0 732 133 in each case disclose a filter of two-stage construction with two separate vertically arranged filter chambers for separating water and particles from fuels. In each case the first filter chambers contain elements for coalescer water drops and the second filter chambers contain fine filters for particle separation. The filter chambers are connected to one another by means of a horizontally extending flow-routing component.
It is the object of the present invention to provide a device for separating water and for filtering impurities out of liquid hydrocarbons or fuels, whereby the effectiveness of water separation is improved and installation space is optimized at relatively low costs.